The long-range research goals of this project are to understand the genetic events that accompany lymphocyte differentiation and that specify lymphocytes function. In the proposed studies, the mechanism by which the immunoglobulin heavy chain (IgH) enhanver controls Ig gene expression in B lymphocytes will be investigated. In particular, the hypothesis that the enhancer's role is in the establishment but not in the maintenance of high level Ig expression will be tested. The physical relationship between IgH enhancer sequences and gene sequences found at nuclear matrix attachment sites will be explored, with reference to the role both kinds of DNA sequences play in Ig gene expression. In these experiments, an endogenous IgH gene that lacks the IgH enhancer element, and yet is expressed at high levels, will be examined for evidence of a matrix attachment site. Studies recently completed in this laboratory implicated a T cell- derived negative regulatory factor in the observed cell-type specificity of the IgH enhancer. Further studies will be undertaken to define the site and mode of action of this negative regulator. Experiments will also be initiated to both map (by chromosome association) and to physically isolate (by DNA- mediated gene transfer) the gene(s) that encodes this T cell factor(s).